As an example of a device for sufficiently changing a position of a predetermined part of the seat, a known seat apparatus is disclosed in U.S. Pat. No. 6,095,610. The disclosed seat apparatus includes a link mechanism for changing a position of a footrest relative to the seat cushion. Such link mechanism basically includes a known four joint linkage having four links provided in a rectangle and rotation shafts for connecting the links. The four joint linkage applied to the part which is sufficiently changed also includes plural auxiliary links for securing the strength of the link mechanism enough to support a load applied to the footrest even if an angle between the links becomes equal to or more than a predetermined angle.
A structure of the link mechanism having aforementioned configuration is complicated because many links and rotation shafts connecting the links are provided thereto. In addition, relatively large and heavy links and rotation shafts need to be provided to such link mechanism applied to a vehicle seat apparatus to secure enough strength for supporting excessive load applied to the seat.
A need exists for a seat apparatus to include a link mechanism being light in weight and having few links.